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Significant changes in atmospheric CO 2 over glacial-interglacial cycles have mainly been attributed to the Southern Ocean through physical and biological processes. However, little is known about the contribution of global biosphere productivity, associated with important CO 2 fluxes. Here we present the first high resolution record of Δ 17 O of O 2 in the Antarctic EPICA Dome C ice core over Termination V and Marine Isotopic Stage (MIS) 11 and reconstruct the global oxygen biosphere productivity over the last 445 ka. Our data show that compared to the younger terminations, biosphere productivity at the end of Termination V is 10 to 30 % higher. Comparisons with local palaeo observations suggest that strong terrestrial productivity in a context of low eccentricity might explain this pattern. We propose that higher biosphere productivity could have maintained low atmospheric CO 2 at the beginning of MIS 11, thus highlighting its control on the global climate during Termination V.

Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking.Here we present species-specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 and 188C, where Mg/Ca 5 1.006 6 0.032 * e 0.065 6 0.003 * Tiso (R 2 5 0.82). G. bulloides is found to calcify deeper in the Southern Ocean ($ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between $ 21 and 98C, of the form Mg/Ca 5 0.580 6 0.016 * e 0.084 6 0.006*Tiso (R 2 5 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1r propagated error in Mg/Ca-derived temperatures is 1.18C for G. bulloides and 0.98C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.

Global biosphere productivity is the largest uptake flux of atmospheric carbon dioxide (CO 2 ), and it plays an important role in past and future carbon cycles. However, global estimation of biosphere productivity remains a challenge. Using the ancient air enclosed in polar ice cores, we present the first 800,000-year record of triple isotopic ratios of atmospheric oxygen, which reflects past global biosphere productivity. We observe that global biosphere productivity in the past eight glacial intervals was lower than that in the preindustrial era and that, in most cases, it starts to increase millennia before deglaciations. Both variations occur concomitantly with CO 2 changes, implying a dominant control of CO 2 on global biosphere productivity that supports a pervasive negative feedback under the glacial climate.

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<p>The primary production, or oxygenic photosynthesis of the global biosphere, is one of the main source and sink of atmospheric oxygen (O<sub>2</sub>) and carbon dioxide (CO<sub>2</sub>), respectively. There has been a growing number of evidence that global gross primary productivity (GPP) varies in response to climate change. It is therefore important to understand the climate- and/or environment controls of the global biosphere primary productivity for better predicting the future evolution of biosphere carbon uptake. The triple-isotope composition of O<sub>2</sub> (Δ<sup>17</sup>O of O<sub>2</sub>) trapped in polar ice cores allows us to trace the past changes of global biosphere primary productivity as far back as 800,000 years before present (800 ka). Previously available Δ<sup>17</sup>O of O<sub>2</sub> records over the last ca. 450 ka show relatively low and high global biosphere productivity over the last five glacial and interglacial intervals respectively, with a unique pattern over Termination V (TV) - Marine Isotopic Stage (MIS) 11, as biosphere productivity at the end of TV is ~ 20 % higher than the four younger ones (Blunier et al., 2012; Brandon et al., 2020). However, questions remain on (1) whether the concomitant changes of global biosphere productivity and CO<sub>2</sub> were the pervasive feature of glacial periods over the last 800 ka, and (2) whether the global biosphere productivity during the “lukewarm” interglacials before the Mid-Brunhes Event (MBE) were lower than those after the MBE.<br>Here, we present an extended composite record of Δ<sup>17</sup>O of O<sub>2</sub> covering the last 800 ka, based on new Δ<sup>17</sup>O of O<sub>2</sub> results from the EPICA Dome C and reconstruct the evolution of global biosphere productivity over that time interval using the independent box models of Landais et al. (2007) and Blunier et al. (2012). We find that the glacial productivity minima occurred nearly synchronously with the glacial CO<sub>2</sub> minima at mid-glacial stage; interestingly millennia before the sea level reaches their minima. Following the mid-glacial minima, we also show slight productivity increases at the full-glacial stages, before deglacial productivity rises. Comparison of reconstructed interglacial productivity demonstrates a slightly higher productivity over the post-MBE (MISs 1, 5, 7, 9, and 11) than pre-MBE ones (MISs 13, 15, 17, and 19). However, the mean difference between post- and pre-MBE interglacials largely depends on the box model used for productivity reconstruction.</p>

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<p>High precision measurements of triple isotopic composition of oxygen in water is a useful tool to infer the dynamic of past hydrological cycle when measured in ice core together with δ<sup>18</sup>O and δD. In particular, the triple isotopic composition of oxygen in water provides information on the climatic conditions of the evaporative sources. In parallel, it has been shown that the triple isotopic composition of oxygen in the atmospheric dioxygen can be a useful tracer of the global biosphere productivity and hence reconstruct the dynamic of the global biosphere productivity in the past from measurements performed in the air bubbles. Measuring triple isotopic composition of oxygen both in the water and in the atmospheric dioxygen trapped in bubbles in ice cores is thus a strong added value to study the past variability of water cycle and biosphere productivity in parallel to climate change.</p><p>Here, we first present new laboratory experiments performed in closed biological chambers to show how the triple isotopic composition of oxygen in atmospheric dioxygen can be used for quantification of the biosphere productivity with determination of fractionation coefficients. Then, we present new records of triple isotopic composition of oxygen in water and O<sub>2</sub> trapped in bubbles from the EPICA Dome C ice core over the deglaciations of the last 800 ka.</p>